Reactivity mechanistic aspects

In the first chapter, devoted to thiazole itself, specific emphasis has been given to the structure and mechanistic aspects of the reactivity of the molecule most of the theoretical methods and physical techniques available to date have been applied in the study of thiazole and its derivatives, and the results are discussed in detail The chapter devoted to methods of synthesis is especially detailed and traces the way for the preparation of any monocyclic thiazole derivative. Three chapters concern the non-tautomeric functional derivatives, and two are devoted to amino-, hydroxy- and mercaptothiazoles these chapters constitute the core of the book. All discussion of chemical properties is complemented by tables in which all the known derivatives are inventoried and characterized by their usual physical properties. This information should be of particular value to organic chemists in identifying natural or Synthetic thiazoles. Two brief chapters concern mesoionic thiazoles and selenazoles. Finally, an important chapter is devoted to cyanine dyes derived from thiazolium salts, completing some classical reviews on the subject and discussing recent developments in the studies of the reaction mechanisms involved in their synthesis. 

TaniaPhos active catalyst discussion As shown by Salzer (2) such complexes with half sandwich stracture result in the catalyst cycle into a hydride species where the pentadienyl moiety can be hydrogenolyticaUy liberated (2, 6). This was verified in the case of BINAP complexes (2, diss. Podewils, Geyser). In accordance to this fact and other mechanistic aspects from Noyori s work (3, 5) it is likely that the pre-catalyst species undergoes the same reaction pathway and that the reactive part of the pre-catalyst, the pentadienyl moiety, will be liberated under hydrogenolytic conditions as shown below in Scheme 23.9 ... 

Aromatic nitro compounds undergo nucleophilic aromatic substitutions with various nucleophiles. In 1991 Terrier s book covered (1) SNAr reactions, mechanistic aspects (2) structure and reactivity of anionic o-complexes (3) synthetic aspects of intermolecular SNAr substitutions (4) intramolecular SNAr reactions (5) vicarious nucleophilic substitutions of hydrogen (VNS) (6) nucleophilic aromatic photo-substitutions and (7) radical nucleophilic aromatic substitutions. This chapter describes the recent development in synthetic application of SNAr and especially VNS. The environmentally friendly chemical processes are highly required in modem chemical industry. VNS reaction is an ideal process to introduce functional groups into aromatic rings because hydrogen can be substituted by nucleophiles without the need of metal catalysts. 

Pyrazolone-1,2-dioxides 311 were subjected to cycloaddition with a wide range of olefinic compounds leading to the 0-2,3,3 ,4-tetrahydro-pyrazolo[l,5-3]isoxazole cycloadducts 312. The behavior of these reactive species 311 toward unsaturated compounds, stereochemical and mechanistic aspects, were discussed in details (Equation 134) <1994J(P2)1337>. 

We will present mechanistic aspects of the Diels-Alder reaction, its selectivity and reactivity in order to explain solvent effects on the one hand, and the effects of Lewis acids on the other. Other catalytic systems like micelles will also be addressed. Some of the explanations may seem trivial or are well-known but, as we will use these in later sections, a clear terminology is desirable. 

Alkynes, although not as prevalent as alkenes, have a number of important uses in synthesis. In general, alkynes are somewhat less reactive than alkenes toward many electrophiles. A major reason for this difference in reactivity is the substantially higher energy of the vinyl cation intermediate that is formed by an electrophilic attack on an alkyne. It is estimated that vinyl cations are about lOkcal/mol less stable than an alkyl cation with similar substitution. The observed differences in rate of addition in direct comparisons between alkenes and alkynes depend upon the specific electrophile and the reaction conditions.111 112 Table 4.4 summarizes some illustrative rate comparisons. A more complete discussion of the mechanistic aspects of addition to alkynes can be found in Section 6.5 of Part A. 

This chapter is concerned with reactions that introduce or replace substituent groups on aromatic rings. The most important group of reactions is electrophilic aromatic substitution. The mechanism of electrophile aromatic substitution has been studied in great detail, and much information is available about structure-reactivity relationships. There are also important reactions which occur by nucleophilic substitution, including reactions of diazonium ion intermediates and metal-catalyzed substitution. The mechanistic aspects of these reactions were discussed in Chapter 10 of Part A. In this chapter, the synthetic aspects of aromatic substitution will be emphasized. 

In the earlier volume of this book, the chapter dedicated to transition metal peroxides, written by Mimoun , gave a detailed description of the features of the identified peroxo species and a survey of their reactivity toward hydrocarbons. Here we begin from the point where Mimoun ended, thus we shall analyze the achievements made in the field in the last 20 years. In the first part of our chapter we shall review the newest species identified and characterized as an example we shall discuss in detail an important breakthrough, made more than ten years ago by Herrmann and coworkers who identified mono- and di-peroxo derivatives of methyl-trioxorhenium. With this catalyst, as we shall see in detail later on in the chapter, several remarkable oxidative processes have been developed. Attention will be paid to peroxy and hydroperoxide derivatives, very nnconunon species in 1982. Interesting aspects of the speciation of peroxo and peroxy complexes in solntion, made with the aid of spectroscopic and spectrometric techniqnes, will be also considered. The mechanistic aspects of the metal catalyzed oxidations with peroxides will be only shortly reviewed, with particular attention to some achievements obtained mainly with theoretical calculations. Indeed, for quite a long time there was an active debate in the literature regarding the possible mechanisms operating in particular with nucleophilic substrates. This central theme has been already very well described and discussed, so interested readers are referred to published reviews and book chapters . 

The book is arranged in 14 chapters. After discussing general aspects, separation of hydrocarbons from natural sources and synthesis from Ci precursors with the most recent developments for possible future applications, each chapter deals with a specific type of transformation of hydrocarbons. Involved fundamental chemistry, including reactivity and selectivity, as well as stereochemical considerations and mechanistic aspects are discussed, as are practical applications. In view of the immense literature, the coverage cannot be comprehensive and is therefore selective, reflecting the authors own experience in the field. It was attempted nevertheless to cover all major aspects with references generally until the early 1994. 

The mechanistic aspects of aromatic121 and alkene122 radical cation reactions have been reviewed. A second review article covers the structure and properties of hydrocarbon radical cations, as revealed by low-temperature ESR and IR spectroscopy.123 A review of the reactivity of divalent phosphorus radical cations has appeared which discusses ionic and SET processes and their kinetics.124 The structure and reactivity of distonic radical cations have been reviewed, including experimental and calculated heats of formation, structures, reactivity, and mechanisms.122125... 

The alkylation of aromatic hydrocarbons with methyl alcohol over Nafion-H catalysts, including the mechanistic aspects, has been studied in detail. The degree of conversion of methyl alcohol was much dependent on the nucleophilic reactivity of the aromatic hydrocarbon. For example, the reactivity of isomeric xylenes was higher than that of toluene or benzene. 

There is also a continuing series of reviews concerning itself with the mechanistic aspects of the reactivity of co-ordinated ligands in Mechanisms of Inorganic and Organometallic Reactions, Plenum. 

The following subsections are devoted to various mechanistic aspects of the ortho photocycloaddition. The possible role of ground-state complexes will be discussed and, subsequently, the intermediate species that are formed or may be formed upon photoexcitation will be treated the reactive excited state, exciplexes, and zwitterions, biradicals, and ion pairs. Empirical rules, aimed at predicting under what circumstances ortho photocycloaddition (or other modes of addition) may occur, will be discussed next and, finally, the results of theoretical considerations and calculations will be reviewed. 

The main problems related to the use of vinyl or dienyl sulfoxides in cycloadditions can be grouped into two different categories, synthetic and mechanistic aspects respectively. Availability of the starting materials, stability of the obtained cycloadducts, and final elimination of the sulfinyl group, are the three main problems to be solved from a synthetic perspective. Reactivity of the substrates and the endo/exo selectivity of the cycloadditions will remain as important questions to be answered from a mechanistic viewpoint. 

In this review we have gathered the important work on quantitative and mechanistic aspects of electrophilic aromatic reactivity of heterocycles. We have concentrated in particular on acid-catalyzed hydrogen exchange, nitration, and gas-phase elimination, these being the major efforts of our own research groups. However all other electrophilic substitution reactions are covered for completeness. 

Practically, all of the above reactions have been realized, with different metals and conditions. In determining the scope of this review, we have attempted to focus our attention on the nature of the transformations at the metal center, especially with regard to oxidation state and formation of the initial alkyl-, alkoxy-, or carboalkoxy-metal bond from saturated precursors. Therefore, while it appears that hydrocarboxylation reactions make some contribution to the total reactivity in a variety of alcohol carbonylation systems, we feel that the mechanistic aspects of this topic would be better covered separately. So, except for noting where this chemistry makes probable contributions, it will not be discussed here. Similarly, homologation reactions, which are believed to usually proceed by way of aldehyde intermediates, will be discussed only as they pertain to the incorporation of the CO into the metal-carbon bonds, that is, the factors governing the subsequent hydrogenation reactions will not be covered. 

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